The present invention relates to a method of producing hard, scratch-resistant surface coatings by way of the sol-gel technology on a wide variety of polymers.
The production of glass like coatings on steel polymers has been known for a long time and described in numerous instances.
By scratch resistance is meant, here and below, the resistance of the surface coating of a material to the imposition of scratches, which is determined using a mar resistance tester. A disk of defined shape and size is pressed with a defined force—the applied-pressure force, measured in N—onto the material provided with the surface coating, and, with this applied-pressure force maintained, is guided at a defined inclination and in a fluid movement over the surface of the surface coating. This movement is repeated, each time with an increased applied-pressure force, until alteration in the form of a linear mark becomes visible on the surface. The applied-pressure force setting at that point is, in the context of the present invention, the measure of the scratch resistance. In the context of the present invention the apparatus used is the mar resistance tester model 435 from Erichsen GmbH & Co KG, Am Iserbach 14, D-58675 Hemer, Germany.
By abrasion resistance is meant, here and below, the loss in weight suffered by the surface coating of material as a result of abrasion effected under defined conditions. This abrasion is brought about by two abrading wheels, whose outside faces have a defined roughness and which are moved abrasively over the surface coating with a defined number of rotations and with an application force which is defined by the mass of the abrading wheel. The apparatus used in this case is described by and obtainable from Taber Industries (455 Bryant Street, North Tonawanda, N.Y. 14120, USA; http://www.taberindustries.com).
DE 38 28 098 A1 describes the production of scratch-resistant materials starting from organofunctional silanes and at least one aluminium alkoxide. A composition obtained by hydrolytic polycondensation of at least one aluminium compound and at least one organofunctional silane is applied to a substrate and cured by heating. A particular step in the production of the composition is seen as being its necessary precondensation by addition of water in a sub-stoichiometric ratio prior to application of the composition to the substrate. The water must be added in several steps in order to avoid unwanted precipitation.
DE 39 17 535 A1 as well describes the production of compositions for scratch-resistant coatings on the basis of hydrolysable silicon compounds. Besides organofunctional silanes use is made of aluminium alkoxides, titanium alkoxides and/or zirconium alkoxides. To achieve the scratch resistance a volatile fraction of the hydrolysis compounds is removed by evaporation. In this method, again, water is added in a substoichiometric ratio to effect precondensation of the mixture of hydrolyzable silicon compounds, thereby necessitating a separate method step. This precondensation can be accelerated through the use of a condensation catalyst. Only after the precondensation and the removal of volatile hydrolysis compounds by evaporation is the composition applied to the substrate and subsequently cured by heating for a few minutes to two hours.
A combination of alkoxides of Al, Ti and/or Zr and at least one organofunctional silane is described in U.S. Pat. No. 4,746,366. This combination is precondensed by addition of water in stages. The hydrolysis products are removed from the combination under reduced pressure. The resulting product is applied to the substrate and cured by thermal heating for a duration of a few minutes up to two hours.
Coating materials for producing abrasion-resistant protective coats on substrates made of plastic are described in DE 199 52 040 A1 and DE 102 457 25 A1. The coating material disclosed in these specifications must be applied in at least two coats, namely a scratch-resistance coat or primer coat and a top coat, then precondensed and at least partly cured. The precondensation time can be shortened through the addition of a condensation accelerator. Curing the coating formed from these coats takes times of at least 30 minutes at a temperature of 130° C.
DE 40 11 045 A1 describes the production of a scratch-resistant coating to which a commercially available photoinitiator is added. After this coating material has been applied to a plastics substrate, it can be cured thermally or by irradiation with UV light. As a result of this photochemical treatment a short cure time of 120 seconds is achieved.
A feature common to all of these publications is that the production of the coatings and/or coating materials includes a time-consuming step of precondensation. A further disadvantage of the prior art is that the coatings must be applied multiply, up to a coating thickness of more than 15 μm, in order to acquire the desired scratch resistance or abrasion resistance. For coatings of this kind, therefore, it is necessary to expend a relatively high level of time and cost until the coated plastics surface is robust or serviceable. The generally long cure time, of up to 2 hours, is shortened in the prior art only through the use of a photoinitiator, which effects curing by means of UV irradiation. In the known literature there is no information on coatings or coating materials which are produced via a sol-gel technique and can be cured thermally within times of up to 5 minutes.
It was an object of the present invention to provide a method, improved over the prior art, of producing scratch-resistant and abrasion-resistant coatings on polymeric surfaces that does not have one or more disadvantages of the prior art.